The present invention relates to an integrated circuit design for radiation hardening.
Electrons trapped in high earth orbits and electrons and protons trapped in low and medium earth orbits cause a high level of ionizing radiation in space. Such ionizing radiation causes an accumulation of charge in electronic circuits which eventually results in a malfunction or failure of the circuits.
Electron-hole pairs generated in the bulk silicon of an integrated circuit do not present a severe problem, as the electrons and holes recombine rapidly. Electron-hole pairs formed in field oxide of an integrated circuit are more difficult to deal with because the electrons are far more mobile than the holes and may become separated from the holes, interfering with recombination and resulting in an accumulation of net positive charge in the field oxide, or other dielectric film. The edge region between the diffusion region and the field oxide below a polysilicon gate, referred to as the xe2x80x9cbird""s beakxe2x80x9d region, is particularly susceptible to the effect of the ionizing radiation. The accumulation of net positive charge in the field oxide beneath the polysilicon gate can cause leakage of electrons across the gate, turning on the gate prematurely. Even slight leakage across the many gates in a typical integrated circuit can cause excess power drain and overheating of the integrated circuit.
Integrated circuit designs have been developed to withstand high levels of ionizing radiation. Such design methodologies can involve redundancy of electronic circuits, suitable doping of the semiconductor material and spacing of electronic circuits. Such methodologies require increased cost for redesign and production.
In accordance with an aspect of this invention, the active channels of a transistor are completely within the diffusion region of the transistor, thereby minimizing the formation of active channels at edges between the diffusion region and the field oxide. Such edges, referred to as a bird""s beak region, are susceptible to leakage due to ionizing radiation. Those bird""s beak region that do exist, lie across regions of equipotential, minimizing the effect of such leakage. Integrated circuits in accordance with the design of the present invention may be fabricated through ordinary fabrication processes.
In one embodiment, an outer annular transistor and one or more inner annular transistors positioned within the outer annular transistor, are provided. The outer transistor may be used as an active transistor in the formation of logic circuits, or may provide isolation for the one or more inner transistors which may be connected to form logic circuits. The design preferably includes a provision for readily disabling the radiation resistant system so that the same design can be easily transformed into a non-radiation resistant design.
Other types of components used in integrated circuits can be isolated from the effects of ionizing radiation through use of an annular gate electrode, as well. For example, a resistor can be surrounded by an annular electrode to prevent deleterious leakage caused by ionizing radiation.